The role of cyclic AMP is being investigated for its regulation of cellular differentiation and aging in Dictyostelium discoideum. Adenylate cyclase activity was not detected until the aggregation stage of development (10 h) when a sudden peak of activity was found. The enzyme was active at all subsequent stages, although a slow decline in activity was observed. Similarly, cyclic AMP levels were not detectable through the first 7 h of development and then showed a sudden peak at aggregation. Following aggregation the cyclic AMP levels decreased to approximately 1/3 the peak value and maintained that level throughout the remainder of the developmental cycle. 5'AMP nucleotidase activity was found to be localized in undifferentiated prestalk cells. By utilizing a micro-enzymatic technique the activity was found to be present only in a narrow band of cells separating the prestalk and prespore cells. Visualization of the reaction product by electron microscopy demonstrated that the enzyme is located on the plasma membrane with its active site directed extracellularly. We have also used ultramicrotechniques in the biochemical analysis of cyclic nucleotide phosphodiesterase (PD) distribution in individual aggregates at various stages of development. During culmination, activity rose to 40-60 mmol/h/kg associated with the developing stalk, while it declined in the spore mass. Extracellular phosphodiesterase inhibitor produced at the aggregation stage was found to reduce the localized activity in the culmination stage by 50-80%, with the most marked inhibition occurring in the center of the papilla. We found no evidence of endogenous heat-stable phosphodiesterase inhibitor within the culminating sorocarp.